1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive device comprising a ceramic substrate and at least a piezoelectric/electrostrictive element stacked on the ceramic substrate by means of a film formation method, and more particularly to a piezoelectric/electrostrictive device comprising a plurality of piezoelectric/electrostrictive layers and a plurality of electrode layers including a piezoelectric/electrostrictive material stacked alternately in a comb like structure on a ceramic substrate and to a method for producing the same.
2. Description of the Related Art
In a piezoelectric/electrostrictive device such as an actuator element and a sensor element including a piezoelectric/electrostrictive layer, firstly, a wiring pattern, which is composed of one electrode layer, is formed on a ceramic substrate by, printing for example. Secondly, the piezoelectric/electrostrictive layer is further formed on the wiring pattern by printing to secure the wiring pattern and the piezoelectric/electrostrictive layer to the ceramic substrate by sintering. After that, a wiring pattern, which is composed of the other electrode layer, is formed.
The piezoelectric/electrostrictive device is used as an actuator element in which an electric field is applied to the piezoelectric/electrostrictive layer by supplying an electric signal to the wiring pattern, and the piezoelectric/electrostrictive layer is consequently displaced. Additionally, the piezoelectric/electrostrictive device can be used as a sensor element in which an electric signal, which is generated depending on a pressure applied to the piezoelectric/electrostrictive layer, is extracted from the wiring pattern.
The piezoelectric/electrostrictive device as described above involves the following fear. That is, for example, the wiring pattern in the lower layer undergoes any thermal shrinkage, and a part of the wiring pattern is evaporated when the piezoelectric/electrostrictive layer is sintered. As a result, a lot of unnecessary pores appear in the wiring pattern and pores having large opening areas appear to decrease the portion (conductive portion) which actually functions as the electrode layer.
In such a situation, the area of the electrode layer is substantially decreased. Therefore, the capacitance is decreased and the driving force is lowered. Further, when unnecessary pores are generated irregularly due to the dispersion in the production, then the electrode areas of individual devices are dispersed, and the capacitance is also dispersed. This results in decreasing yield of the devices. Further, it is necessary that the control voltage is adjusted for every individual device when the device is used. A problem occurs in that such a device is difficult to be used (difficult to be controlled).
Further, the adhesive force is weak between the wiring pattern and the piezoelectric/electrostrictive layer. Therefore, any exfoliation occurs in some cases during the machining (for example, cutting and polishing) and/or during the washing (washing with ultrasonic wave) of the piezoelectric/electrostrictive device. In the case of the device as described above, the place, at which the exfoliation occurs, is the interface between the wiring pattern and the piezoelectric/electrostrictive layer.
Further, a portion, in which the electrode layer is not formed on the ceramic substrate, is provided in relation to the planar shape of the wiring pattern. At this portion, the ceramic substrate and the piezoelectric/electrostrictive layer are opposed to one another. However, any gap appears at the portion after the sintering, for the following reason. That is, it is difficult to join constitutive materials of the ceramic substrate and constitutive materials of the piezoelectric/electrostrictive layer.
If such a gap is formed, a part of the piezoelectric/electrostrictive layer consequently float over the electrode layer. Such a portion exists as a region which is not restricted by the ceramic substrate. As a result, any movement tends to occur due to any external force in this structure. Therefore, the exfoliation is apt to take place.
In view of the above, in order that the piezoelectric/electrostrictive layer and the wiring pattern are not exfoliated from the ceramic substrate, for example, it is necessary that the step of cutting the piezoelectric/electrostrictive device is performed under a condition in which the load on the piezoelectric/electrostrictive layer or the like is decreased. That is, the cutting step is restricted by the condition in which the machining load is small. As a result, the machining time is prolonged, and the throughput is lowered.
On the other hand, it is necessary that the washing step is also performed under a condition in which the load on the piezoelectric/electrostrictive layer or the like is decreased. Therefore, in order to eliminate the dirt, it is necessary to use a longer period of washing time. As a result, the number of steps is increased.
When the exfoliation of the piezoelectric/electrostrictive layer or the like occurs, the following harmful influences also appear.
(1) The function as the piezoelectric/electrostrictive device is deteriorated.
(2) When the wiring pattern is exfoliated from the piezoelectric/electrostrictive layer, the capacitance is decreased. As a result, the amount of generation of strain is decreased in the piezoelectric/electrostrictive layer, and the displacement is decreased.
(3) When the wiring pattern is exfoliated from the ceramic substrate, then the strain, which is generated in the piezoelectric/electrostrictive layer, is hardly transmitted to the ceramic substrate, and the displacement is decreased.
(4) The strength of the entire piezoelectric/electrostrictive device is decreased, and the resonance frequency is lowered when the piezoelectric/electrostrictive device is used as an actuator element.
The present invention has been made taking the foregoing problems into consideration, an object of which is to provide a piezoelectric/electrostrictive device which makes it possible to increase the occupied area of a conductive portion in one electrode layer, which increases the driving force, which improves the yield, and which is easily controllable, and a method for producing the same.
Another object of the present invention is to provide a piezoelectric/electrostrictive device which makes it possible to effectively reduce the volume of a piezoelectric/electrostrictive element itself to decrease the resistance on the displacement action and which makes it possible to further increase the driving force (increase the displacement amount) in addition to the requirements described above, and a method for producing the same.
Still another object of the present invention is to provide a piezoelectric/electrostrictive device which makes it possible to prevent a piezoelectric/electrostrictive element formed on a ceramic substrate from exfoliation, reduce the number of steps in relation to the production of the piezoelectric/electrostrictive device, and improve the throughput and which also makes it possible to avoid the deterioration of function of the piezoelectric/electrostrictive device, and a method for producing the same.
Still another object of the present invention is to provide a piezoelectric/electrostrictive device which makes it possible to improve the shock resistance by increasing the breaking strength or fracture strength and which has high reliability, and a method for producing the same.
According to the present invention, there is provided a piezoelectric/electrostrictive device comprising a ceramic substrate and at least a piezoelectric/electrostrictive element stacked on the ceramic substrate; wherein the piezoelectric/electrostrictive element includes a plurality of piezoelectric/electrostrictive layers and a plurality of electrode layers stacked alternately in a comb like structure on the ceramic substrate, the electrode layer includes one or more intermediate electrode layers at an intermediate portion of the piezoelectric/electrostrictive element, the intermediate electrode layers are formed by sintering a cermet film containing a conductive material and a piezoelectric/electrostrictive material, and in at least one of the intermediate electrode layers, the conductive material shrinks in the sintering and forms a conductive portion occupying 80% or more of the intermediate electrode layer.
The area described above is herein defined as follows. That is, the maximum straight line lengths of the openings of the individual pores and the number (n) of the pores are measured within a range of a square having an area of 100 xcexcm2 on the electrode surface at a magnification of xc3x97200 by using a metallographic microscope. Assuming that the shapes of the pores are circular, the pore area is calculated on condition that an average value (r) of the maximum straight line lengths of the openings is an average diameter. The calculated value is multiplied by the number (n) of the pores to obtain a multiplied value from which a pore area ratio (h) per 1 mm2 is calculated. The expression of calculation is shown below.
h=nxcfx80(r/2)2xc3x97100/1 mm2
Usually, if the electrode layer is sintered singly or together with the piezoelectric/electrostrictive layer to secure the electrode layer to the piezoelectric/electrostrictive layer, then a large number of pores are generated, and/or pores having large opening areas are generated, for example, due to the partial evaporation and/or the thermal shrinkage of the electrode layer during the sintering. It is feared that the portion (conductive portion), which actually functions as the electrode layer, may be decreased. It is noted that the pores generated as described above are filled with the piezoelectric/electrostrictive material.
However, in the present invention, the intermediate electrode layer, which is disposed at the intermediate portion in the piezoelectric/electrostrictive element, is formed by sintering the cermet film containing the conductive material and the piezoelectric/electrostrictive material, and the area after the sintering of the conductive portion of one of the intermediate electrode layers disposed at the intermediate portion in piezoelectric/electrostrictive element occupies 80% or more of the intermediate electrode layer. Therefore, the unnecessary pores as described above are generated to a smaller extent.
Therefore, it is possible to increase the occupied area of the conductive portion of one electrode layer. Accordingly, the capacitance is increased, and the driving force is increased, and thus the displacement amount is increased as well.
The unnecessary pores are scarcely generated in the present invention as compared with a case in which the unnecessary pores are irregularly generated. Therefore, the dispersion of the area of the conductive portion of one electrode layer in the individual device is also decreased. Accordingly, the dispersion of capacitances among the individual devices is decreased. It is unnecessary that the control voltage is adjusted for every device one by one when the device is used. Thus, the device is conveniently usable (easily controllable).
Similarly, the dispersion of displacement characteristics of the individual devices is also decreased. Thus, it is possible to improve the accuracy in relation to the displacement amount.
The electrode layer included in the electrode layers for constituting the piezoelectric/electrostrictive element, which is disposed at the intermediate portion in the stacking direction, is formed by sintering the cermet film containing the conductive material and the piezoelectric/electrostrictive material. Therefore, a state is given, in which the electrode layer and the piezoelectric/electrostrictive layer are hardly exfoliated from each other.
Therefore, in the present invention, it is possible to avoid the exfoliation of the piezoelectric/electrostrictive element formed on the ceramic substrate, it is possible to reduce the number of steps in relation to the production of the piezoelectric/electrostrictive device, and it is possible to improve the throughput. Additionally, it is possible to avoid the deterioration of the function as the piezoelectric/electrostrictive device as well.
It is preferable that a range of a volume ratio between the conductive material and the piezoelectric/electrostrictive material is 4:6 to 9:1. Further, it is preferable that the intermediate electrode layer is formed as a conductor layer which is a film of 4 xcexcm or less in thickness. The volume ratio herein refers to the value which is calculated from masses and specific gravities or densities (provided that pores are excluded from values) of materials when the materials are blended.
In this arrangement, it is possible to thin the thickness of one electrode layer. Therefore, it is possible to effectively decrease the volume of the piezoelectric/electrostrictive element itself as well. Accordingly, it is possible to decrease the resistance on the displacement action, and it is possible to further increase the driving force (increase the displacement amount) in cooperation with the increase in capacitance.
It is preferable that the intermediate electrode layer is formed as a cermet conductor layer which is a film of 4 xcexcm or more in thickness. In this arrangement, even when the cermet electrode is 4 xcexcm or more and thick, it is possible to enhance the adhesive force with respect to the piezoelectric/electrostrictive layer as compared with a conductor layer made of a metal simple substance having the same thickness. Therefore, the exfoliation scarcely occurs, and this arrangement is advantageous to improve the reliability.
When the electrode layer, which is disposed at the intermediate portion in the stacking direction, is formed by sintering the cermet film containing the conductive material and the piezoelectric/electrostrictive material, the metal electrode layer is formed at a portion interposed between the piezoelectric/electrostrictive layers after the sintering. That is, the thin electrode layer based on metal is formed at the portion interposed between the piezoelectric/electrostrictive layers, because the piezoelectric/electrostrictive material component in the cermet film is moved to the piezoelectric/electrostrictive layer during the sintering, and the remaining metal material forms the electrode layer. In other words, the metal electrode layer is formed to be thinner than the cermet electrode layer. The electrode layer of the present invention is thin, it has a small number of pores, and it has the high adhesive force with respect to the piezoelectric/electrostrictive layer, as compared with a case in which the electrode layer is formed of a metal paste.
Further, it is preferable that one or more gaps of one or more lower electrode layers positioned at a lower portion of the piezoelectric/electrostrictive element is formed on the ceramic substrate and the gaps are filled with an insulating layer. In this arrangement, the insulating layer and the piezoelectric/electrostrictive layer formed as the upper layer of the insulating layer are strongly bonded. Accordingly, a state is given, in which the electrode layer and the insulating layer are hardly exfoliated from each other. Therefore, it is possible to avoid the exfoliation of the piezoelectric/electrostrictive layer formed on the electrode layer, and it is possible to sufficiently exhibit the function of the actuator element or the sensor element based on the use of the piezoelectric/electrostrictive layer.
It is also preferable that one or more electrode layers provided at an upper portion of the piezoelectric/electrostrictive element, are formed by depositing a film of resinate of a conductive material to the upper portion.
According to another aspect of the present invention, there is provided a piezoelectric/electrostrictive device comprising a ceramic substrate and a piezoelectric/electrostrictive element formed on the ceramic substrate; wherein the ceramic substrate includes fixed sections which have a large thickness and a pair of thin plate sections which are formed continuously from the fixed sections and each of which have a thickness thinner than that of the fixed sections; and each of the pair of thin plate sections are composed of two or more types of materials.
Accordingly, it is possible to increase the breaking strength or fracture strength of the pair of thin plate sections, and it is possible to improve the shock resistance. This results in the improvement in reliability of the piezoelectric/electrostrictive device.
It is also preferable to provide a piezoelectric/electrostrictive device comprising a ceramic substrate and a piezoelectric/electrostrictive element formed on the ceramic substrate; wherein the ceramic substrate includes fixed sections which have a large thickness and a pair of thin plate sections which are formed continuously from the fixed sections and which are thinner than the fixed section; and a second material is used between the pair of thin plate sections and the fixed sections.
Accordingly, the breaking strength is increased by the second material between the pair of thin plate sections and the fixed section. Thus, it is possible to improve the shock resistance of the entire piezoelectric/electrostrictive device.
When the ceramic substrate has a fixed section which has a large thickness, a pair of thin plate sections which are formed continuously from the fixed sections and which are thinner than the fixed sections, and movable sections which are provided at ends of the pair of thin plate sections, it is also preferable that a second material is used between at least the pair of thin plate sections and the fixed section and between the pair of thin plate sections and the movable sections.
Accordingly, the breaking strength is increased by the second material used between the pair of thin plate sections and the fixed sections and between the pair of thin plate sections and the movable sections. Thus, it is possible to improve the shock resistance of the entire piezoelectric/electrostrictive device.
In this arrangement, it is also preferable that the second material is used over ranges from between the pair of thin plate sections and the fixed sections to between the pair of thin plate sections and the movable sections. Accordingly, it is also possible to enhance the strength of portions of the pair of thin plate sections other than the joined portions.
It is also preferable that the second material is a metal. The ceramics have a high breaking strength against the compressive stress, but the breaking strength of the ceramics is low against the tensile stress. On the other hand, the metal has the high breaking strength against the tensile stress, but the breaking strength of the metal is low against the compressive stress. Therefore, when the two materials (ceramics and metal) are combined, it is possible to mutually supplement the drawbacks of both, and it is possible to secure high strength.
Further, when the metal is arranged on the surface, the metal causes elastic deformation in response to the tensile stress, and the metal absorbs the stress. Therefore, the fracture limitation is raised, and the breaking strength is increased as compared with a case in which the ceramics is exposed to the surface. In particular, it is possible to enhance the shock resistance.
It is also preferable that the second material is a cermet. It is preferable that the metal having a high elastic modulus is directly arranged on the ceramics. However, there is such a possibility that the metal may be exfoliated. Therefore, it is appropriate to use the cermet which has a high joining strength with respect to the ceramics.
It is preferable that the second material is a cermet composed of a constitutive material of the ceramic substrate and a metal. In this arrangement, when the ratio of the metal in the cermet is low, then the joining strength with respect to the ceramics is increased, but the property as the metal becomes poor. Therefore, it is preferable to select a condition in which the ratio of the metal is high and it is possible to secure the joining strength with respect to the ceramics.
It is also preferable that the piezoelectric/electrostrictive element is arranged on at least one thin plate section of the pair of thin plate sections.
According to still another aspect of the present invention, there is provided a method for producing a piezoelectric/electrostrictive device comprising a ceramic substrate and a piezoelectric/electrostrictive element formed on the ceramic substrate; the method including a step for manufacturing the ceramic substrate by sintering a molding after forming the molding by using raw materials containing a ceramic material; a step for forming the piezoelectric/electrostrictive element by a plurality of piezoelectric/electrostrictive layers and a plurality of electrode layers stacking alternately in a comb like structure on the ceramic substrate; wherein the step for forming the piezoelectric/electrostrictive element includes a step for forming one or more intermediate electrode layers at an intermediate portion of the piezoelectric/electrostrictive element by sintering a cermet film containing a conductive material and a piezoelectric/electrostrictive material to form the intermediate electrode layers.
Accordingly, it is possible to increase the driving force, it is possible to improve the yield, and it is possible to obtain the piezoelectric/electrostrictive device which is easily controllable. Further, it is possible to avoid the exfoliation of the piezoelectric/electrostrictive element formed on the ceramic substrate, it is possible to reduce the number of steps in relation to the production of the piezoelectric/electrostrictive device, and it is possible to improve the throughput. Additionally, it is also possible to avoid the deterioration of the function as the piezoelectric/electrostrictive device.
In this procedure, it is also preferable that the step for forming said piezoelectric/electrostrictive element includes a step for filling one or more gaps of one or more lower electrode layers positioned at a lower portion of the piezoelectric/electrostrictive element with an insulating layer to form the lower electrode layers.
It is also preferable that after a first cermet to be converted into the lower electrode layers and a second cermet to be converted into the insulating layer are formed on the ceramic substrate by printing respectively, the first and second cermets formed on the ceramic substrate by printing are sintered.
According to still another aspect of the present invention, there is provided a method for producing a piezoelectric/electrostrictive device comprising a ceramic substrate including fixed sections which have a large thickness and a pair of thin plate sections which are formed continuously from the fixed sections and which has a thin thickness, and a piezoelectric/electrostrictive element formed on the ceramic substrate; the method including a step for forming a cermet paste by printing on opposing surfaces of a plurality of ceramic green sheets to be converted into the thin plate sections; a step for laminating the plurality of ceramic green sheets to form a ceramic green laminate; a step for sintering the ceramic green laminate to form a ceramic laminate; and a step for cutting off unnecessary portions after forming and sintering the piezoelectric/electrostrictive element on the ceramic laminate to manufacture the piezoelectric/electrostrictive device in which a second material is used between the pair of thin plate sections and the fixed sections.
Accordingly, it is possible to produce the piezoelectric/electrostrictive device which makes it possible to increase the breaking strength of the pair of thin plate sections and which makes it possible to improve the shock resistance. Thus, it is possible to realize the high reliability of the piezoelectric/electrostrictive device.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.